Desibel

Overview and links

The background for this project was the plan by Nordic Mining to open a rutile mine at a mountain called Engebøfjellet, at a small place called Vevring. This mine would give the locals a 24/7 noise concert for at least 30 years, in addition to polluting the fjord with dust from the mine. Only 2% of the output would be rutile, which can be refined to titanium. A few people (Wenche Wefring and Torkil Sandsund) decided to protest against this by making a sound sculpture for the 30th anniversary of the Vevring Exhibition. The mountain is shown right behind the church in the picture below.

Early in 2009 I was contacted to help with the design of this sound sculpture, which was to be a giant horn. It was to be built by Geir Hjetland, and the music would be a noise composition by Maja Ratkje.

Technical

The Concept

The idea was to make a visually spectacular loudspeaker, which should also be able to play as loud as possible, to make the aural and visual parts match. For this we needed both power and efficiency.

To the left is the "Desibel Team", from left to right: Torkil Sandsund, Bjørn Kolbrek, and Geir Hjetland.

Design of the Big Horns

The first task was to design the largest horn. The original idea was to use three huge horns in parallel, but we had to reduce the size enough for the horn to fit through the doors of Geir Hjetland's workshop, and also to make sure it would be possible to transport the horn without a big truck. The horn was to be built from empty juice barrels, welded together. This determined the throat area. The driver was selected after consultation with Robin Stöckert of Soundscape Studios in Trondheim.

Simulations were run to determine a suitable profile. Due to the large throat area, long length and relatively small mouth of the horn, the cutoff was low, and the mouth was too small for proper impedance matching. The horn therefore has a quite resonant response. But as this design was intended to be used as an instrument that would be an integral part of the music composition, it would not be a big problem.

The horn is 9.5m long and the mouth diameter is 2.4m. The resulting profile and simulated response is shown to the left.

Building the horns

The horns were built by artist Geir Hjetland. It was a challenge even for this experienced metal worker.

The horns were built from steel juice barrels, welded together. First as a long tube, that was later split up, and the gaps filled with pieces from other barrels.

In addition to making the horn, Geir also built a skeleton rack to support the horn, especially for the transport, but also to hold it in place and to make it easier to move around.

Midrange and Tweeters

To provide a full frequency range, we also needed horns for the midrange and treble. For the midbass/midrange, we used a pair of 12" drivers in 90Hz exponential horns, about 90x90cm mouth area, and 1m long. For tweeters, we used a pair of 2" compression drivers in some store-bought constant directivity horns.

Amplifiers and mounting

As the "preamp" we used a small Mackie mixer. This mixer feeds a Behringer DCX2496 digital crossover, which again feeds the power amplifiers. For the tweeters we use Alto Mac2.2, the midrange uses Citronic Pro2000, and the bass horns use Citronic Pro3200.

To transport the whole system, we have a custom made trailer, built by a Norwegian trailer manufacturer. I won't tell you which one, since they wouldn't sponsor us... :)

All the equipment will fit on the trailer, but we transport the electronics in the car.

The Music

The music for Desibel was a specially written 10 minute long piece by Maja Ratkje. Based on my simulations of the horn, she tuned the fundamental notes of the sounds to match the resonance peaks of the horns. This way we could take advantage of the highest efficiency ranges for making sound.

Testing, summer 2009

When we finally had received all the parts, and all the horns were welded and mounted, we assembled it outside Geir's workshop, and measured it.

It's not easy to measure a horn this big, because the near field is quite large. Phasing is also a difficult point, and where do you measure to get the phasing right? We ended up measuring the individual horns to get an idea on the usable frequency range, and then adjusted things mostly by ear. Crossover frequencies: 100Hz and 1kHz.

The measurements shown (the curves have very little smoothing):

Black: the entire setup, about 4m from the big horn's mouth. Red: The response of the big horn. The resonances are at about the same frequencies as in the simulation. Blue: The response of the small horn. The horns supplement each other, giving a pretty smooth final curve.

Upgrades and maintenance

In a high-power system like this, the components get pretty stressed. We have had to replace most of the drivers. The midbass drivers were replaced by a more powerful variant, and we added an extra pair of tweeters for a total of 4. We have also added an extra amplifier, so that each bass horn now has its own bridged power amp.

Desibel in use

Vevring Exhibition, 2009

This was the first real performance, and it was enthusiastically received by the locals. When standing at the right spot in Vevring, you could hear how the sound from the horns echoed back from over the fjord.